A display device which is much thinner and lighter than a conventional cathode-ray tube, namely a flat panel display has been developed. A liquid crystal display device having a liquid crystal element as a display element, a display device having a self-light emitting element, an FED (a field emission display) utilizing an electron source, and the like are competing against each other as a flat panel display, and are desired to consume low power and have a high contrast ratio to be differentiated from other products by adding a high value.
In a general liquid crystal display device, each substrate is provided with one polarizing plate, whereby a contrast ratio is kept. As a contrast ratio becomes high, clearer black display can be performed, whereby a high display quality can be provided in a case where a picture is seen in a dark room like a home theater.
For example, a structure is proposed, in which a third polarizing plate is provided in a case where a first polarizing plate is provided on an outer side of a substrate on a viewing side of a liquid crystal cell, a second polarizing plate is provided on an outer side of a substrate opposite to the viewing side, and light from an auxiliary light source provided on the substrate side is polarized through the second polarizing plate to pass the liquid crystal cell, in order to increase a contrast ratio (refer to Patent Document 1: PCT International Publication No. 00/34821).
As a flat panel display like a liquid crystal display device, there is a display device having an electroluminescent element. The electroluminescent element is a self-light emitting element and no light irradiation means such as a backlight is required, whereby thinning of a display device can be attempted. Further, a display device having an electroluminescent element has an advantage that response speed is higher and dependence on a viewing angle is less than a liquid crystal display device.
A structure is also proposed, in which a polarizing plate or a circularly polarizing plate are provided, with respect to a display device having an electroluminescent element (Patent Document 2: Japanese Patent No. 2761453 and Patent Document 3: Japanese Patent No. 3174367).
As a structure of a display device having an electroluminescent element, a structure is proposed, in which light emitted from a light emitting element interposed between light transmitting substrates can be observed as light on an anode substrate side and light on a cathode substrate side (Japanese Published Patent Application No. H10-255976).